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Exclusive Excerpt: How the Body Fuels Exercise
This post was written by Chris Beardsley of Strength and Conditioning Research. The views expressed herein are his.
We're entering a genuinely new phase in fitness, where personal trainers and athletes are reaching for science to help them identify the best methods for getting fitter, losing weight, or getting more muscular. If you've never delved into the science of fitness before, where can you start? At the outset, it can appear overwhelming. Recently, Streng
Greatist is pleased to bring you a sample review from the Background collection. If you’ve ever been curious about how the aerobic and anaerobic systems work together, this is a good place to start, to find out what we know and what we still have to find out.
How do the different energy systems work together during exercise?
In this study, published in the journal Sports Medicine, Paul Gastin from the Victorian Institute of Sport, Melbourne, Australia, describes how the aerobic and anaerobic energy systems interact to provide the energy we need during exercise .
How does the body provide energy to the cells?
Gastin explains there are three energy systems in the body, each of which is designed ultimately to provide adenosine triphosphate (ATP) to cells as the main form in which energy is provided. These energy systems are:
- Splitting phosphocreatine anaerobically
- Anaerobic breakdown of carbohydrate by glycolysis
- Aerobic or oxidative metabolism
The reviewer explains these three energy systems are capable of handling diverse energy demands, and they interact smoothly to restore ATP supplies during and following exercise.
What did scientists originally think about the way energy was provided in the body?
Gastin explains the interaction and relative contribution of the energy systems was first investigated during the 1960s and 1970s. These researchers developed the ideas of an energy continuum, which suggested that the type of energy system used depended on the type of exercise that was being performed.
They thought the phosphocreatine system was used primarily during short, sharp efforts, like throwing a ball. They suggested the anaerobic system was mostly responsible for events lasting for a few minutes, and they thought the aerobic system supplied all the necessary energy when activity went on longer than that.
However, on account of varying measurement methods, these early researchers produced very different estimates of the relative contribution of the three systems to exercise of increasing durations. Some estimated aerobic and anaerobic systems supplied energy equally after two minutes, while others estimated this point occurred after four minutes.
What was wrong with their measurement methods?
The reviewer explains it's easy to quantify the contribution of the aerobic system to energy release, as it's directly related to the amount of oxygen absorbed through the breathing process. This can be measured by monitoring inspired and expired gases.
However, he also notes it's much harder to quantify the energy released through anaerobic systems, as only cellular processes are involved and there is still currently no universally accepted measurement method for this. Previously used methods include the measurement of oxygen debt, testing blood lactate measures, and monitoring power output.
What do we know about anaerobic energy supply?
Gastin explains that during short-duration, high power outputs, the energy is mainly supplied by the two anaerobic systems: breakdown of phosphocreatine and anaerobic glycolysis.
The reviewer explains peak rates for ATP synthesis from both systems appear to supply energy equally over the first 6 seconds. Interestingly, while the creatine phosphate system reaches its peak output at 1.3 seconds and declines from there, the glycolytic system does not reach a peak until after 5 seconds have passed.
And what do we know about aerobic energy supply?
Gastin explains that when exercise is performed at a work rate below the anaerobic threshold, the maximum volume of oxygen that is taken in per minute (VO2 max) increases exponentially before reaching a steady state. However, during short bursts of very intense exercise (30 – 90 seconds), the volume of oxygen taken in per minute only reaches 90 percent of VO2 max. Exactly why the aerobic system does not operate at its highest level during maximal exercise is not known.
Gastin goes on to note researchers have obtained varying estimates of the aerobic contribution to the commonly used, 30-second Wingate Test. And these estimates range from 13 – 44 percent. Similar attempts have been made for other athletic tasks, such as the 400 and 800 meter runs, both of which entail an anaerobic and an aerobic component. Currently, measurement methods do not permit a definitive answer. However, there is a clear trend for increasing aerobic contribution with increasing exercise duration, as can be seen in the following chart.
So how do the anaerobic and aerobic energy systems work together?
Based on the reviewer's analysis of a number of studies investigating the relative contributions of the anaerobic and aerobic energy systems to intense exercise, there appears to be an equal contribution from the aerobic and anaerobic energy systems at between 1 – 2 minutes, which is much sooner that previous researchers concluded.
At the end of the study, Gastin proposes that the aerobic contribution during intense exercise has been consistently underestimated. Indeed, he further suggests that the cross-over point from predominantly anaerobic to predominantly aerobic energy supply may occur as soon as 20–30 seconds following starting exercise. However, we cannot be certain at this stage.
What did the reviewer conclude?
The reviewer concludes that work by early scientists gave rise to two common misconceptions. The first is that the energy systems operate during exercise in a sequential manner. The second is that the aerobic system responds slowly to energy demands and plays little role in short-duration exercise.
Instead, Gastin explains that the aerobic system is very involved in short-duration exercise and responds quickly, such that it is the dominant energy pathway after just 30-seconds. And secondly, he explains all three energy pathways are constantly operating but simply providing proportionally different contributions to total energy supply.
What does it mean for you?
Current research suggests anaerobic and aerobic energy systems are all constantly operating but providing proportionally different contributions to total energy supply. Most physical activities are therefore likely to derive some energy from each of the three systems.
Therefore, we should not neglect training that will develop all three systems for optimal fitness or for sports performance and we should accept that short duration training can play a significant role in developing aerobic fitness.
Want more information like this? "Background" includes over 50 reviews and covers important concepts in muscular, neuromuscular, bone, tendon, and cardiovascular physiology as well as their adaptations to training. It also has sections on energy systems, fatigue, and biomechanics.
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